Apparatus and method for treating FGD blowdown or similar liquids

ABSTRACT

A process has steps of one or more of aerobic treatment to remove COD and nitrify a waste stream, anoxic treatment to denitrify a waste stream, anoxic treatment to remove selenium and anaerobic treatment to remove heavy metals and sulphur. The process may be used to treat, for example, FGD blow down water. The process may further include one or more of (a) membrane separation of the waste stream upstream of the anoxic digestion to remove selenium, (b) dilution upstream of the biological treatment step, (c) physical/chemical pretreatment upstream of the biological processes or dilution step to remove TSS and soften the waste stream, or (d) ammonia stripping upstream of the biological treatment steps or dilutions step. These processes may be provided in a variety of suspended growth or fixed film reactors, for example a membrane bioreactor or a fixed film reactor having a GAC bed. Processes for biological treatment of inorganic compounds in a fixed medium reactor is described including steps of one or more of maintaining desired ORP levels, optionally by controlling nutrient addition, and removing solids or gas bubbles from the medium bed.

This is a continuation of International Application No.PCT/CA2006/001220, filed Jul. 24, 2006, which is an application claimingthe benefit under 35 USC 119(e) of U.S. application Ser. Nos. 60/701,996filed Jul. 25, 2005; and 60/736,859 filed Nov. 16, 2005; and,PCT/CA2006/001220 claims priority to Canadian Patent Application No.2,517,322 filed Aug. 26, 2005. All applications listed above areincorporated herein, in their entirety, by this reference to them.

FIELD

This invention relates to water treatment including biological watertreatment and treatment of feeds containing inorganic contaminants, forexample selenium, nitrates or heavy metals, for further example scrubberblow down water from a flue gas desulfurization (FGD) operation in acoal fired power plant.

BACKGROUND

The following background discussion does not imply or admit that anyprocess or apparatus described below is prior art or part of theknowledge of people skilled in the art in any country.

Scrubber blow-down water from a flue gas desulfurization operation in acoal-fired power plant contains a wide range of inorganic contaminantsremoved from the flue gas. The blow down water may also contain organiccontaminants, such as di basic acid (DBA), and ammonia added as part ofor to enhance the FGD process. The FGD scrubber blow-down water may havevery high total dissolved solids where the main anions are chlorides andthe main cations are calcium, magnesium and sodium. The rate ofblow-down may be controlled to maintain a desired chloride concentrationcausing the blow-down water to have a high, but generally stablechloride concentration. The concentration of other contaminants may varywidely as influenced, for example, by burning coal from differentsources even in a single power plant. However, the concentration of TDS,TSS, Ca and Mg hardness, nitrate, ammonia, and sulfur for example assulphate are all likely to be high, and various heavy metals may bepresent, making the blow down water very difficult to treat,particularly to achieve very low levels of contaminants. Otherwastewaters, such as wastewater discharged from mining operations,agricultural drainage or run off water, other industrial waters or evendrinking water, may also have unacceptable concentrations of some or allof these inorganic contaminants.

Current methods of treating blow down water rely heavily on physical andchemical processes to remove inorganic contaminants. The physical andchemical processes involve costly chemicals and produce large amounts ofsludge. Arsenic, mercury and heavy metals may also be present in theblow down water at above regulated levels. Further, some jurisdictionshave recently regulated selenium concentrations in effluents dischargedto the environment. The permitted concentration of selenium may be 0.5ppm or less or 200 ppb or less while the blow down water may contain1-20 or 2-10 ppm of selenium which is not removed in conventionaltreatment plants.

In U.S. Pat. No. 6,183,644, entitled Method of Selenium Removal andissued on Feb. 6, 2001 to D. Jack Adams and Timothy M. Pickett,dissolved selenium is removed from contaminated water by treating thewater in a reactor containing selected endemic and other seleniumreducing organisms. Microbes may be isolated from the specific water orimported from other selenium contaminated water. The microbes are thenscreened for ability to reduce selenium under the site specificenvironmental conditions. The selected microbes are optimized forselenium reduction, then established in a high density biofilm within areactor. The selenium contaminated water is passed through the reactorwith optimized nutrient mix added as needed. The elemental selenium isprecipitated and removed from the water. Products using this or asimilar process may be available under the trade mark ABMet® fromApplied Biosciences Corp of Salt Lake City, Utah, U.S.A. The entirety ofU.S. Pat. No. 6,183,644 is incorporated herein by this reference to it.

SUMMARY

The following summary is intended to introduce the reader to one or moreinventions described herein but not to define any of them. Inventionsmay reside in any combination of one or more of the apparatus elementsor process steps described anywhere in this document.

It is an object of an invention described herein to improve on, or atleast provide a useful alternative to, the prior art. It is an object ofan invention described herein to provide a wastewater treatment processor apparatus. Other objects of one or more inventions described hereinare to provide an apparatus or process for treating FGD blow down wateror other wastewaters having selenium or nitrate or both, or a process orapparatus for biologically removing inorganic contaminants, for examplenitrogen, selenium, arsenic, mercury or sulphur, from waste water. Thewastewater may be a raw wastewater or a wastewater that has beenpretreated.

A process is described herein having steps of anoxic treatment todenitrify a waste stream, anoxic treatment to remove selenium andanaerobic treatment to remove heavy metals or sulphur or both. Removalof heavy metals is possible because SO₄ is present and converted tosulfide by anaerobic SO₄ reducing bacteria. The process may furtherinclude one or more of (a) membrane separation of the waste streamupstream of the anoxic digestion to remove selenium, (b) dilutionupstream of the biological treatment step, (c) physical/chemicalpretreatment upstream of the biological processes or dilution step toremove TSS and soften the waste stream, for example through the additionof lime or sulfides and the removal of precipitates, (d) ammoniastripping upstream of the biological treatment steps or dilution step or(e) aerobic treatment to remove COD and nitrify the waste streamupstream of the anoxic treatment. Some of the biological treatment stepsmay be performed in a fixed film reactor, for example a granularactivated carbon bed. One or more of the biological treatment steps mayalso be performed in a suspended growth reactor such as a membranebioreactor. Each biological treatment step may be performed in adistinct reactor optimized to perform a step or two or more of thebiological treatment steps may be performed in a multiple purposereactor.

An apparatus is described herein having one or more reactors configuredto provide nitrification, denitrification, selenium and heavy metalsremoval and sulphur removal by biological treatment. The apparatus mayfurther have a reactor to provide aerobic treatment of COD. The reactorsmay include a membrane bioreactor or a fixed film reactor. The fixedfilm reactor may comprise a bed of activated carbon or other supportmaterials. The apparatus may further have one or more of an inlet fordiluting the feed water to the biological processes, a system for addinglime or sulfides to the wastewater upstream of the biological reactors,one or more physical or chemical pretreatment systems, a precipitateremover, or an ammonia stripper.

An apparatus or process are also described herein for removing nitratesin a fixed film bioreactor. An apparatus or process are also describedherein for providing a fixed film bioreactor having two or more zones.The zones may be adapted to remove different contaminants and may havediffering oxygen reduction potential (ORP). An apparatus or process arealso described herein for using nutrient location or feed rate tocontrol ORP in a fixed film or media bioreactor. An apparatus or processare also described herein for bumping a fixed film or media bioreactorto release solids or gas bubbles from a media bed.

A process or apparatus described herein may be used for treating FGDblow down water or pretreated FGD blow down water to produce an effluentwith low concentrations of selenium, for example 1 ppm or less or 10 ppbor less, and low concentrations of total nitrogen, for example 1 mg/L orless or 10 ppm or less, in the effluent. However, an apparatus orprocess described herein may also have applications in treating blowdown water when selenium concentration in the effluent is not a concern.An apparatus or process described herein may also be useful for treatingother wastewaters having selenium or nitrate, for example mining, oragricultural runoffs or waste streams, contaminated ground or surfacewater streams, or petroleum refinery waste streams, particularly wherethe waste stream also has significant concentrations of one or more ofCOD, nitrate, ammonia, TDS, TSS, hardness, CaSO₄, or sulphate.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of embodiments of one or more inventions will be describedbelow with reference to the Figures described below.

FIG. 1 is a schematic process flow diagram of an apparatus and processfor treating water.

FIG. 2 is a schematic process flow diagram of another system fortreating water.

FIG. 3 is a schematic diagram showing an alternate embodiment for partof the system and process of FIG. 1 or 2.

FIG. 4 is a cross section of a bioreactor.

FIG. 5 is a chart of experimental results using a multi-stagebioreactor.

FIG. 6 is a chart of experimental results relating number of seleniumreducing organisms to temperature.

FIG. 7 is a chart of selenium reduction in FGD blow down water using DBAas a carbon source.

FIG. 8 is a chart showing ranges of ORP for various reactions.

FIG. 9 is a schematic process flow diagram of another system fortreating water.

DETAILED DESCRIPTION

Various apparatuses or processes will be described below to provide anexample of an embodiment of each claimed invention. No embodimentdescribed below limits any claimed invention and any claimed inventionmay cover processes or apparatuses that are not described below. Theclaimed inventions are not limited to apparatuses or processes havingall of the features of any one apparatus or process described below orto features common to multiple or all of the apparatuses describedbelow. It is possible that an apparatus or process described below isnot an embodiment of any claimed invention. The applicants, inventorsand owners reserve all rights in any invention disclosed in an apparatusor process described below that is not claimed in this document and donot abandon, disclaim or dedicate to the public any such invention byits disclosure in this document.

Table 1 shows the contaminants, and their concentrations, assumed forFGD scrubber blow-down water in the design of an example of an apparatusand process described further below. FGD blow-down water may exist withother contaminants or other concentrations of contaminants. Thecomposition of FGD blow-down can also vary widely over time for aspecified coal-fired power plant as influenced, for example, by changesin the source of coal. However, FGD blow-down water is generallycharacterized by very high total dissolved solids (TDS) where the mainanion is chloride and the main cations are calcium, magnesium andsodium. The blow-down water also contains significant concentrations offine suspended solids, including CaSO₄ fines. The blow-down alsocontains a wide range of inorganic contaminants, including ammonia,which is added for selective catalytic reduction in the scrubbingprocess. The blow-down water may also contain some organics,particularly DBA (dibasic acid) which may have been added to enhancescrubber efficiency. In the apparatus and process described below, theeffluent is intended to have a total nitrogen (TN) content of 10 ppm orless and selenium concentrations of 0.4 ppm or less.

TABLE 1 Typical FGD Blowdown Water Parameter Typical Value Min-MaxChlorides 30,000 ppm 20-40,000 ppm pH >5.0<6.0 TDS 75,000 mg/L50,000-150,000 mg/L TSS 2% dry wt 1-5% dry wt Aluminum - Total 960 ppm80-3700 ppm Antimony 12 ppm 0.03-49.0 ppm Ammonia - N 31 ppm 0.25-64 ppmNitrate - N 350 ppm 200-450 ppm Total Nitrogen 200 ppm 50-400 ppmArsenic - Total 15 ppm 0.27-100 ppm Barium - Total 100 ppm 2.0-770 ppmBeryllium 2.1 ppm 0.06-6.9 ppm Boron — 20-40 ppm Cadmium - Total 0.8 ppm0.12-1.5 ppm Calcium 18,000 ppm 10,000-30,000 ppm Chromium - Total 23ppm 0.5-210 ppm Chromium VI — 3-12 ppm Cobalt — 0.05-4 ppm Copper -Total 1.7 ppm 0.3-6.6 ppm CO₃/HCO₃ 1500 ppm 1-3,000 ppm Fluoride 360 ppm61-1600 ppm Iron - Total 1400 ppm 116-6400 ppm Lead - Total 19 ppm0.2-140 ppm Lithium — 2-3 ppm Magnesium 15,000 ppm 10,000-20,000 ppmManganese 10 ppm 3.6-200 ppm Mercury - Total 0.38 ppm 0.5-1.4 ppmNickel - Total 10 ppm 0.5-74 ppm Phosphate - Total 1.0 ppm 0-10 ppmPotassium 6800 ppm 5000-10,000 ppm Selenium - Total 17 ppm 1.5-100 ppmSilver - Total 10.0 ppm 0.002-20 ppm Sodium 15,000 ppm 10,000-20,000 ppmSulfate (SO₄) 60,000 ppm 40,000-80,000 ppm Thallium 0.76 ppm 0.02-2.2ppm Vanadium — 1.0-11.0 ppm Total Zinc 15.0 ppm 1.7-50.0 ppm Temperature130° F. 125-130° F.

In greater detail, treating the blow-down raises several challengeswhich a process or apparatus may address in a variety of ways, asdescribed generally below and then by describing one or more examples oftreatment systems and processes. In the following description,pre-treatment refers to treatment occurring upstream of biologicalprocess steps.

High TDS concentrations make it difficult to maintain activity forbiological treatment. This issue may be addressed by diluting the wastestream upstream of biological treatment. The high TDS also makes itdifficult to flocculate and settle biomass in an activated sludgeprocess. This issue is addressed by using fixed film bioreactors ormembrane bioreactors which may operate with TDS concentrations at whichsettlability is low.

In general, high hardness causes scaling due to Ca or Mg oversaturationand any pH or temperature shifts may cause precipitation of calcium ormagnesium sulfates or carbonates. FGD and other industrial waste watercan contain high levels of sulfate, calcium and magnesium resulting in adanger of scaling conditions, which are exacerbated with increasingalkalinity. When nitrate is removed by biological denitrification,alkalinity is produced which tends to encourage scaling in someprocesses. These issues may be addressed by a softening pre-treatment,for example lime softening, and optionally by adding acid for pHadjustment upstream of the biological process. Scaling may also oradditionally be addressed by performing biological denitrification in afixed film reactor as described below. pH may decrease in the directionof feed flow through such a reactor, thus making such reactors resistantto scaling. In particular, selenium reducing or other microbespopulating the reactor may produce organic acids from the fermentationof the nutrients that are fed to the system, thereby neutralizingalkalinity and inhibiting scaling.

The high TSS, particularly because it is essentially inorganic, causesproblems with developing and controlling a suspended biomass and withbioreactor and membrane plugging. This issue may be addressed bypre-treating the waste stream to coagulate or flocculate and thenphysically remove (for example by settling or floating) suspendedsolids.

High nitrate concentrations are a concern because nitrate is a preferredelectron acceptor for biological reduction over selenate. This issue maybe addressed by decreasing the nitrate concentration upstream of aselenate reducing step. The nitrate reducing step may occur in anupstream part of a selenate reducing reactor, as part of a multi-stepbiological process or multi-part fixed film reactor upstream of aselenate reducing process or part, or as part of a distinct process orreactor.

Ammonia in the blow down water is a concern because concentration in thefinal effluent may be regulated and because oxidation of ammonia mayincrease nitrate concentration. This issue is addressed by removing theammonia, for example, by stripping the ammonia as NH₃ in a pre-treatmentprocess or by removing ammonia biologically by anitrification/denitrification process either in a series process or withrecirculating flows. Denitrification, either as a denitrification stepin a nitrification/denitrification process or to remove nitratesotherwise created by oxidizing ammonia may be performed in a fixed filmbioreactor which may be part of a single or multipart selenium reducingbioreactor.

The presence of various heavy metals, for example Cu, As or Hg, orrelated oxidized contaminants are a concern because they may beregulated in the effluent but are difficult to remove in lowconcentrations. This issue may be addressed in part by precipitatingthese elements out in a pre-treatment softening step. The issue may befurther addressed by biologically reducing SO₄ and precipitating thesecontaminants as metal sulfides after removing nitrate and selanate andselenite. This precipitation may occur in a fixed film bioreactor whichmay be part of a single or multipart selenium reducing bioreactor.

The presence of selenium, as selenate or selenite, is a concern becauseof recent regulation of selenium concentrations in the effluent directlyor indirectly, for example through fish tissue concentrations in thereceiving body. The selenium is difficult to remove because of its lowconcentration and its tendency to form selenate or selenite and dissolvein water making physical or chemical removal difficult, costly orinefficient. Selenium is addressed in the process and apparatus bybiologically reducing it to elemental selenium and then precipitating itfor removal. This may be done in a fixed film bioreactor which may alsobe used to remove other contaminants.

FIG. 1 shows a treatment system 10 having a pretreatment area 12upstream of a biological treatment area 14. Feed 16, which may be FGDblow-down water or another feed, flows into pretreatment area 12. In thepretreatment area 12, a large portion of the TSS in the feed is removedand Ca and Mg are removed to soften the feed 16. The pretreatment area12 uses physical/chemical methods to treat the feed 16. For example,lime or sulfides or both may be added to the feed 16 to precipitatecalcium, magnesium and metals. The precipitates may be removed byclarifiers, for example single stage or double stage clarifiers.Settling can be enhanced by the addition of coagulants or polymers.

Pre-treatment effluent leaves the pretreatment area 12 through apretreatment effluent line 20. Dilution water 18 is added to thepretreatment effluent. The dilution water 18 reduces the total dissolvedsolids (TDS) concentration of the pretreatment effluent to make itacceptable for biological treatment downstream. Sufficient dilutionwater 18 may be added to make the TDS concentration like that ofseawater, for example with a TDS of 35 g/L or less. Any low TDS watercan be used for dilution water 18, for example cooling circuit blow downwater from a power plant. The dilution water 18 also cools the FGDblow-down water, for example from 50° C. or more to about 43° C. or lessor 40° C. or less, to further facilitate biological treatment. As shownin FIG. 6, the health of selenium reducing organisms declines aboveabout 43° C. In general, a fixed film reactor may be operated attemperatures of about 40° F. to about 105° F. An established reactor mayalso tolerate higher temperature for short periods of time, for examplea temperature to 110° F. for up to 24 hours.

The diluted pretreatment effluent then flows to the biological treatmentarea 14. The biological treatment area 14 has four zones: an aerobiczone 22; a first anoxic zone 24; a second anoxic zone 26; and, ananaerobic zone 28. These zones 22, 24, 26, 28 are shown connected inseries in FIG. 1 although one or more of them may alternately beconnected with recirculation loops. Further alternately, some of thezones 22, 24, 26, 28 may not be required in some embodiments. The zones22, 24, 26, 28 may also occur in separate reactors or one or more zones22, 24, 26, 28 may be combined into a single reactor. One or more ofnutrient streams 30, 32, 34 may be used to add nutrients to any zone 24,26, 28 downstream of a nutrient stream 30, 32, 34, either directly orthrough intervening zones 24, 26. For example, nutrients may be added instream 30 or stream 32 to support the growth of bacteria in zones 26 orzone 28 or both.

The aerobic zone 22 is used to nitrify, assimilate or remove ammonia, tothe extent that ammonia has not been stripped in the pretreatment area12, and to oxidize organic carbon. An optional supplemental aerobic zonemay also be added downstream of the anaerobic zone 28 to remove residualnutrients added before or in zones 24, 26, 28 and to oxidize residualcompounds from anaerobic zone 28. If there is no TN discharge limit forthe effluent, or if ammonia is stripped in the pretreatment area 12 suchthat TN in the effluent will be acceptable, the aerobic zone 22 may beomitted, or replaced by an aerobic zone downstream of the anaerobic zone28.

In the first anoxic zone 24, nitrate acts as a preferred electronacceptor and is removed by denitrification. The nitrate may be removedto a concentration which facilitates the biological reduction ofselenium in the second anoxic zone 26, considering that nitrate in highconcentration will be used as an electron acceptor over lowconcentrations of selenate or selenite.

For example, NO₃ may be reduced to 10 mg/L as N or less or 1 mg/L as Nor less or 10 ppm as N or less in the stream leaving the first anoxiczone 24.

In the second anoxic zone 26, selenium is removed by biologicalreduction and removal, for example by precipitation into flush flowwater or waste sludge. These steps may occur, for example, according tothe process described in U.S. Pat. No. 6,183,644 or in other fixed orsuspended bed reactors. The reactors may be seeded with seleniumreducing organisms.

In the anaerobic zone 28, sulfate-reducing bacteria reduce sulfates andproduce sulfides in the form of H₂S or HS⁻. Part of the HS⁻ may reactwith soluble metals to form insoluble metal sulfides which mayprecipitate out of solution. In this way the anaerobic zone removesheavy metals. The off gas from the anaerobic step 28 can be recycled tothe aerobic step 22 or to a downstream aerobic step to reduce theproduction of odors associated with H₂S.

In general, the zones 22, 24, 26, 28 may be arranged into one or morereactors. Each zone 22, 24, 26, 28 may occupy its own reactor, forexample a CSTR optimized to reduce the primary target contaminant ofeach zone 22, 24, 26, 28. Alternately, for example, zones 22 and 24 canbe combined into a combined nitrification/denitrification reactor whichmay have 1, 2 or more tanks. Zones 24, 26 and 28 or 26 and 28 may becombined into an ABMet or other fixed film reactor having one or moretanks. Other reactors may also be used. For suspended growth reactors,the limited concentrations of the target contaminants may be low and thepresence of other contaminants may make biomass separation difficult andso membrane bioreactors are preferred. Alternately, fixed film reactorsmay be used, for example Moving Bed Bioreactors, for example as producedby Anox Kaldnes of Norway, fluidized bed reactors, for example asproduced by Shaw Envirogen of New Jersey, USA, biofilters as produced byDegremont of France under the trade mark BIOFOR, granular activatedcarbon reactors, for example as produced by the Applied BiosciencesCorp. of Utah, USA under the ABMet trade mark, or in membrane supportedbiofilm reactors (MSBR) as described in PCT Publication Nos. WO2004/071973 or WO 2005/016826. Depending on the zone, the MSBR mayoperate autotrophically or heterotrophically optionally using a processof heterotrophic denitrification as described in Canadian PatentApplication No. CA 2,477,333. Membrane separation may optionally be usedwith or after any fixed film reactor although there may also be no needfor it. The entire disclosures of each of WO 2004/071973; WO 2005/016826and CA 2,477,333 are incorporated herein by this reference to them.

FIG. 2 shows a process flow diagram of a treatment plant 50 used for,for example, treating FGD blow-down as described in Table 1. In thepretreatment area 12, pretreatment is by lime softening, optionally withsulfide addition, and 1 or 2 stage settling in clarifiers. Such aprocess may be provided by WesTech of Utah, USA or others. PH isadjusted in the pretreatment effluent 20 when the dilution water 18 isadded to a pH of less than 8.5, for example between 6 and 8 to enhancebiological treatment. In the aerobic zone 22, a membrane bioreactorhaving an aeration tank and a membrane tank containing ZeeWeed™membranes from GE Water and Process Technologies, Zenon MembraneSolutions of Ontario, Canada, may be used for nitrification. The aerobiczone 22 may alternately be omitted. The first and second anoxic andanaerobic zones 24, 26, 28 may be provided in an ABMet® reactor systemby Applied Bioscience Corp. of Utah, USA, or a similar or modifiedsystem which may consist of a 2 or more stage reactor configuration.This reactor is an up-flow fixed film reactor using a GAC bed operatedin plug flow so that 3 biological zones corresponding to the firstanoxic, second anoxic and anaerobic zones 24, 26, 28 can be establishedin sequence. A down flow reactor may alternately be used. A singlenutrient stream 30 may be used upstream of the ABMet reactor.Alternately, if a two or more stage reactor is used, an inlet fornutrients may be provided upstream of one or more of, or each, stagealthough nutrients will not necessarily be provided from each inlet atall times. Precipitates are removed from this reactor by periodicallyflushing the GAC bed to overflow troughs. Sludge from the pretreatmentarea 12 and biological treatment area 14 is fed to a sludge thickenerand dewaterer. Thickened and dewatered sludge is sent to waste. Sludgethickening and dewatering effluent is returned to an equalization tankto be mixed with the FGD blow down feed water 16. Further discussion ofthe use of an ABMet process or apparatus, or media filter generally,which may include modifications or enhancements to such a process orapparatus, are described below.

An ABMet® reactor, may be described as an upflow or downflow, simplefixed growth or fluidized, activated carbon or other support mediabased, biological filter system or bioreactor. A sample reactor 100 isshown in cross section in FIG. 4. A media bed 101 is provided on which apopulation of selected microorganisms will grow and be retained withinthe system. Activated carbon may be employed as the medium and providesa very large surface area available for microbial growth. The activatedcarbon may be provided, for example, in the form of granular activatedcarbon or pelletized activated carbon. Other media might be used, forexample polymeric fibers, crushed stone, pumice, sand, plastic media orgravel. With activated carbon, much of the surface area is protected increvices within each carbon particle, thus sheltering biomass from shearand abrasive forces. The media bed 101 may occupy a zone h2 of thereactor 100.

The specific gravity of an activated carbon particle with attachedbiomass is slightly greater than 1.0. The particles may have a settlingvelocity in still water of about 160 ft/hr. The inventors have noticedthat that settling rate is similar to that of granular sludge in upflowanaerobic sludge blanket (UASB) systems and so the influent distributionsystem and hydraulic regime within the carbon bed during normal forwardflow conditions may be similar to UASB systems.

The reactor may have an upper port 106 and a lower port 102 and abackwash port 103, each of which may be connected to a distribution orcollection system 105, for example one or more horizontal pipes.Optionally, for some systems, lower port 102 and backwash port 103 maybe both connected to the same distribution or collection system 105 orbe replaced by a single port and distribution or collection system 105.Uniform orifices may be located on the underside of these pipes atproper spacing intervals. Orifices may be designed at about a 1.0 psipressure drop, which renders friction losses along each pipeinsignificant and assures even flow distribution. Multiple pipes may beconnected together through various headers or manifolds. One or moregrades of aggregate 104 may be installed around and above thedistribution or collection system or systems 105 in a subfill layer h1.The aggregate 104 will aid in flow distribution while also preventingbreak through of media to a distribution or collection system 105.System 105 connected to upper port 106 may be covered in a screen orhave small holes to prevent media entry if required. Wastewater mayenter the media bed 101 from the top or bottom. Wastewater may enter thereactor 100 through upper port 106, flow downwards through the media bed101 and be withdrawn from lower port 102. Alternately, wastewater mayenter through the bottom port 102, flow upwards through the media bed101 and exit through the upper port 106. Upflow velocity under normalforward flow conditions may be maintained at about 5 ft/hr, well belowthe activated carbon settling rate of 160 ft/hr.

During the course of normal operation, solids will accumulate within themedia bed 101. These solids may be of one or more of three types, 1) TSSwhich enters the reactor 100 and is retained, 2) biomass that is fixedto the media that grows and occupies additional space, and 3) inorganiccontaminants that are biologically converted to solid forms and retainedwithin a bed.

As solids accumulate, the pressure drop across the media bed 101 willincrease. At a selected time interval or pressure point, the media bedmay be flushed. This operation may be accomplished by utilizing backwashport 103 and its associated distribution or collection system 105. Inthis case, an upflow velocity of about 80 ft/hr of feed or water may bemaintained to flush the media bed 101. Other velocities may be used, forexample as in a range that would be used in activated carbon fluidizedbed systems.

The upflow velocity applied during flushing may result in an upwardexpansion of the bed by up to 30% into a bed expansion layer h3. Withthis velocity, media particles may be fluidized, resulting in dislodgingof trapped influent TSS, excessive biomass growth attached to the mediaand associated inorganic contaminants that have been removed from orprecipitated out of the wastewater. The upflow velocity used is stillcomfortably below the settling rate of the media particles. Thus,unacceptable amounts of media will not be flushed, if any, from thebioreactors during this operation. The flushing water and entrainedsolids may be removed through troughs 108. Upper port 106 or lower port102, whichever is used to withdraw treated effluent, may be closedduring flushing.

A headspace layer h4 is provided above the expansion layer h3 and belowa reactor cover 109 in layer h5. Gases released by the microorganismsmay collect in the headspace layer h5. A vent or gas outlet 110 in theheadspace h4 or cover h5 may be used to release these gases to theatmosphere or collect them for further treatment, for example in adownstream branch process or by recycle to an upstream part of thesystem.

From a hydraulic design standpoint, the reactor 100 may operate underconditions similar to UASB systems during upward feed flow periods, ifany, and similar to biological fluidized beds during flushing. Withproper function of up-stream treatment steps, and given the low growthrate of the biomass within the media bed 101, flushing may be requiredfrom between once every two weeks to only a few times each year, forexample once a month. Flushing may be a batch operation over a 30 minuteperiod. Spent flush water may be returned to the head of the plant fromwhere, for example, the solids will settle in solids contact clarifiersand ultimately be co-mingled with primary solids for dewatering anddisposal. Alternately, flush water may be separately treated to separateout solids for separate post-treatment or disposal of the solids sincethe solids flushed from reactor 100 may contain selenium and heavymetals.

In addition to, or alternately in place of, flushing as described above,the reactor 100 may be “bumped”, or briefly fluidized or backwashed,periodically or from time to time. Bumping may be done by flowing water,for example reactor 100 feed water, cooling water or tap water, or amixture of gas, for example air, bubbles and water, through the backwashport 103 at a rate greater than the normal upwards flow feed rate. Thebumping may be done at a velocity of between about 1 gpm/ft² to 14gpm/ft² for a short period of time, for example 10 minutes or less or 5minutes or less. The bumping may be done at a frequency between, forexample, once per hour to once per week, for example between 1 and 4times per day. The bumping expands the media bed 101, allows some solidsto be removed from the system and also releases gas bubbles thataccumulate in the media bed 101. Gas bubbles or solids in the media bed101 can impede flow through the media bed 101. The primary purpose ofbumping is thus to control head loss and provide a more even flow ofwater through the entire media bed 101 particularly by the release ofgas bubbles from the media bed 101. The bump may raise the water levelin the reactor to above troughs 108. Bump effluent water collected introughs 108 can be handled as described for flush effluent. Thisprovides for removal of TSS and may reduce or eliminate the need formore rigorous flushes as described above. Alternately, the effluent fromsome or all of the bumps may be recycled to the lower port 102. Whilethis may place solids in an otherwise undesired part of reactor 100, thebump flows are small and the recycled solids may be removed in fullflushes as described further above. Further, a portion of the solids inthe bump effluent is biodegradable. Recycling these solids may allow fortheir biodegradation and so reduce the total amount of sludge that needsto be wasted from the system as a whole. The desired bumping frequencyand duration is related to the removal of gases or solids and so isrelated to the suspended solids concentration or nitrogen level or bothin the inlet to the reactor 100.

Referring to FIG. 9, another system 200 is shown for treating a feed 202which may be FGD blowdown water. Feed 202 is pretreated in a firstpretreatment apparatus 204 and, optionally, a second pretreatmentapparatus 206 or more pretreatment steps. These apparatus 204, 206 maybe, for example, physical or chemical or physical and chemical treatmentsteps such as combined mixer and clarifier units used, for example, forlime or sulfide precipitation or any of the pretreatment steps describedin this document. Pretreatment effluent 208 flows to a two stagebioreactor 210 having two of the reactors 100 of FIG. 4 operated indownflow configuration in series. Pretreatment effluent 206 flows toupper port 106 of the first reactor 100, then from lower port 102 of thefirst reactor 100 to the upper port 106 of the second reactor and out ofthe lower port 102 of the second reactor. Treated effluent 212 leavingthe second reactor 100 may optionally flow to an aerobic post-treatmentapparatus 214 to produce final effluent 216. Reactors 218 may be flushedor bumped by flowing backwash water from backwash feed tank 218 to thebackwash port 103 of the reactor 100. Bump or flush water collected introughs 108 may be treated in a backwash liquid/solid separationapparatus 220, such as a pond or clarifier. Clarified backwash water maybe returned to the head of the plant. All sludge may be sent to a commonsludge disposal system 224 or the backwash water sludge may be sent to aseparate toxic sludge disposal system 226. Nutrients may be added to thepretreatment effluent 208, the pipe between the lower port 102 of thefirst reactor 100 and the upper port 106 of the second reactor 100 orboth.

The microorganisms that will perform the various functions discussedabove will require a food or carbon source. A molasses-based nutrientmixture may be used. The nutrient may be chosen to provide a carbon:nitrogen: phosphorous ratio (CNP) of, for example, 100:10:1, when mixedwith the feed. For example, the feed may already contain sufficientphosphorous and so there may be no need for phosphorous in the nutrientsolution. Nutrient may be supplied, for example, at a rate of 0.2-0.4gallons of nutrient per 1000 gallons of feed water. This basic mixturemay be supplemented with micronutrients and other components shown topromote stable growth for the target microbial population. The nutrientcan be added to the lead bioreactor and/or to the second or later cellin a two or more stage reactor.

DBA, if used in the FGD system to enhance sulfur removal, will leave aconcentration of DBA in the wastewater. Some or all of the DBA willremain in the wastewater after physical or chemical pretreatments andeven to some extent after an aerobic biological treatment step, if thereis any. DBA is carbon-based and may be used by the microbes as anutrient. As shown in FIG. 7, DBA in sufficient quantities can be usedas the sole carbon source for selenium reducing microorganisms. DBApresent in lesser amounts may be accounted for in reducing the nutrientcarbon amount or feed rate. An excess of DBA, if inhibiting the growthof desired organisms compared to undesired organisms or interfering withORP gradient control, can be reduced by oxidation upstream of thereactor 100. In general, the extent to which DBA may be utilized in theanoxic or anaerobic sections of the beds can be managed by redox ornutrient control or upstream processes.

When reactor 100 is used to provide first and second anoxic zones 24, 26and anaerobic zone 28, there are three primary, biochemical reactions,which may occur within the biomatrix.

-   -   Remaining nitrates are reduced to nitrogen gas, which will be        subsequently released to the atmosphere.    -   Selenium is reduced from an oxidized state to elemental form.        Elemental selenium will form so-called “nanospheres” that will        attach to the cell walls of the microbes performing the        reductive function. As the microbes are attached to the media,        selenium will be likewise retained within the media bed until        flushing.    -   Sulfates present are reduced to hydrogen sulfide. Further, the        sulfides generated in the biomatrix complex as metal sulfide        precipitates that are retained. Sulfides are effective at        complexing with zinc, copper, nickel, lead and a host of other        primary metals. While required metals removal may be        accomplished in the primary treatment steps, the reactor 100 may        provide further treatment and polishing for metals removal.

A bioreactor configuration may involve multiple trains of two or morereactors 100 in series or other configurations. A generally plug flowregime may be maintained as wastewater flows through each train of twoor more successive reactors 100. This arrangement may facilitate controlof redox potential ranges associated with each of the three biochemicalreactions discussed above or others. A single bioreactor 100 can also beused and similarly controlled.

Each of the reactions discussed above occur primarily in different,although possibly overlapping, redox ranges as shown in FIG. 8.Wastewater entering the lead reactor 100 in each train will have apositive or slightly negative redox level. As the flow progressesthrough the bed, the redox level will gradually decrease. As the redoxlevel drops into negative territory, remaining nitrates will be reducedfirst. As can be seen from FIG. 8, nitrates will be reduced at levelsdown to −150 mV.

Selenium will then be reduced as the redox proceeds downward to −200 mV.Lastly, sulfate reduction to sulfide will occur down to −300 mV.Although the ORP range for sulfate reduction and selenium reductionlargely overlap, selenium reduction may occur first and in preferenceover sulfate reduction and so sulfate reduction may not reach highlevels or be complete until after selenium reduction is nearly complete.

With a generally plug flow regime, a redox gradient is developed throughthe bed or beds. This gradient can be controlled by adjusting the rateof nutrient addition or hydraulic retention time (HRT) or both in thebed or beds. In general, HRT may be altered at the design stage bychoosing the bed dimensions in relation to the feed either by changingthe dimensions of the bed or the number of beds in series or parallel.HRT in one or more beds in series may be, for example, in the range from2 hours to 12 hours. After a system is built, changes to feed flow rateto the reactor 100 will alter HRT. However, nutrient addition may beeasier to vary after a system is built and operating. Higher levels ofnutrient addition will drive redox lower; reducing nutrient additionwill cause redox level to rise. Optional inter-stage nutrient additionbetween reactors 100 in series may provide additional flexibility inredox control. ORP sensors may be provided at the outlet of each of thetwo reactors 100 in a two stage system. An ORP sensor may also beprovided in the feed to a reactor 100. ORP sensors may also be providedat intermediate points in one or more stages of a reactor. An operatoror automated system may record the ORP values at a sampling interval andmaintain a record of the values. The values may be used to determine aninput parameter to a controller or decision table. For example, the ORPvalues in the outlet of each of the two stages of a two stage reactormay be recorded at a sampling rate, which may vary from, for example, 1to 24 times per day. A running average of the values over an averagingperiod, for example 0.5 to 2 days is calculated. The running averagesare used and considered in determining, manually or automatically, howthe supply of nutrients upstage of either or both of the stages shouldbe altered to produce a more desired redox level.

By being able to control redox level, the system can be tuned tooptimize removal of certain contaminants. Specifically, nutrientaddition can be controlled to maintain optimal redox for seleniumreduction (−50 to −200 mV) in at least a zone of the reactor. Forexample, nutrient addition upstream of a first reactor 100 in a twostage system may be calculated to produce an ORP of −50 to −200 mV atthe outlet of the first reactor. Nutrient addition upstream of a secondreactor 100 may be controlled to provide an ORP of −200 to −350 mV inthe outlet of the second reactor 100 to enhance sulfide production inthe second reactor 100. As an alternate example, in an application wheremetals removed in the reactor is less important, nutrient addition maybe controlled to provide an ORP of −50 mV or less, for example −50 mV to−100 mV, at the outlet of the first reactor 100 and −200 mV or more, forexample, −150 mV to −200 mV at the outlet of the second reactor 100 of atwo stage system.

The generally plug flow regime allows specific populations to reside insomewhat dedicated zones within the carbon beds and so each one maycontain multiple types of organisms although the redox levels forvarious functions may overlap. Nonetheless, removal can be achieved fora range of contaminants.

The influence of ORP gradient on contaminant removal is demonstrated inFIG. 5. Water entering a reactor 100 contains dissolved oxygen, andoxidized species of contaminants (i.e., nitrate, selenate, selenite),and has a positive ORP of +100 in this example. Samples were collectedat various points across a multi-stage bioreactor and measured for ORP,selenium, sulfate, and nitrate-N. Contaminants were removed in theirrespective stages along this ORP gradient that had been established inthe bioreactor bed.

ORP can be controlled through the system by inter-stage nutrientdelivery, and by controlling the HRT of each stage, which is done byincreasing or decreasing the influent flow rate. The ORP can becontrolled within the range of +150 mV to −400 mV.

In the example of FIG. 5, a reactor was divided into six stages.Nitrate-N is removed in the first stage, with only 25% of the seleniumbeing reduced. None of the sulfate is reduced. Removal in later stagesis as shown in the Figure. Through controlling the ORP gradient, asystem can be configured to remove for, for example, only Nitrate-Nremoval, Nitrate-N removal with selenium removal, and/or multiplecontaminant removal via direct reduction and sulfide precipitation.

Denitrification may be carried out concurrent with or before sulfate andselenium reduction. As the biomass carrying out these reactions arefixed to the support media (which will be retained within the system),the evolution of nitrogen gas will not materially impact biomassretention. Gases created in a reactor may be removed by bumping asdescribed above as required.

The biomass that will grow in the bioreactor cells may occur naturallyin our environment. Sulfate reducers and denitrifiers may grow naturallywith little encouragement required. Given that required metals removalmay be accomplished in the primary treatment steps, control of redoxlevels may optionally be such that sulfate reduction (and sulfideformation) is minimized.

Microbes that have demonstrated the ability to reduce oxidized seleniumto elemental form have been isolated from various locations in thewestern U.S.A., for example by Applied Biosciences Corp. of Utah U.S.A.Several species of these microorganisms may be isolated and grown. Atplant start-up, an innoculum charge of these microbes may be supplied toseed the bed. For example, the bioreactor 100 may be seeded with aninitial charge of microbes. The initial charge may contain a mixture of2-6 strains of microbes of the genus Pseudomonas, Shewanella,Alcaligenes or other environmental microbes, which have been selectedbased on, or in anticipation of, their growth and contaminant reductionin the water of interest. A population large enough to seed the reactor100 may be grown on site from a supply of centrifuged and freeze driedstarter microbes prepared remotely. These microbes quickly attach toactivated carbon or other media, and proliferate thereafter in thepresence of nutrient material. Following innoculum loading, thebioreactor cells may be operated in a recycle mode for several days toallow the microbes to attach. An alternate seeding procedure is to firstsoak the bed in a nutrient solution. After this pretreatment,approximately one reactor value of a mixture of seed microorganisms inwater, optionally containing some nutrient solution, is placed in thereactor. The reactor is then allowed to sit for a period of time, forexample 1 to 3 days, to allow the microorganisms to attach to the mediabefore starting feed flow through the reactor. Thereafter, normal feedflow can be introduced and plant commissioning may proceed.

Following the completion of commissioning, a periodic bioassayevaluation may be conducted. This evaluation may involve collection ofcarbon samples at various depths in each bioreactor cell. Each samplemay be measured for redox level and the microbes examined to develop abiological community profile. This study may generate an informationbaseline to ensure that the proper microbial mix is maintained withinthe system over time and may be considered, for example in combinationwith on-line ORP measurements or to adjust target ORP ranges, incontrolling nutrient addition.

Two or more bioassay evaluations may be conducted followingcommissioning of a plant. In the event that a bioassay evaluation showsthat the specific microbes needed for selenium reduction are not presentin appropriate numbers, a supplemental inoculum charge may be provided.Additional bioassay evaluations may be performed from time to time tomonitor the performance of the reactor and the results may be consideredin modifying desired ORP ranges at one or more points in the reactor. Areactor as discussed above may be used to provide one or more of thefirst and second anoxic and anaerobic zones 24, 26, 28.

FIG. 3 shows a nitrification/denitrification reactor 80 used to providethe aerobic and first anoxic zones 22, 24 of FIG. 1.Nitrification/denitrification reactor 80 may also be used to replace thebioreactor tank and ZeeWeed tank of FIG. 2 to provide the aerobic andfirst anoxic zones 22, 42 and allow the ABMet reactor to be operatedwith the second anoxic and anaerobic zones 26, 28 with a minimal or nofirst anoxic zone 24.

1. A process for the treatment of a liquid comprising: a) physical and/or chemical pretreatment of the liquid to remove total suspended solids and to soften the liquid; b) ammonia stripping of the liquid; c) anoxic treatment to remove nitrate; d) a second anoxic treatment after step (c) to remove selenium and anaerobic treatment to remove heavy metals; and, e) membrane separation of the liquid upstream of step (d).
 2. The process of claim 1 further comprising the step of: f) dilution upstream of the ammonia or anoxic steps.
 3. The process of claim 1 wherein the process is carried out in a series of reactors and at least one of the reactors is a membrane bioreactor.
 4. The process of claim 3 wherein one of the reactors is a fixed film reactor and is downstream of the membrane bioreactor.
 5. The process of claim 4 wherein the fixed film reactor has a granular activated carbon bed.
 6. The process of claim 5 the operation of the activated carbon bed reactor comprises a step of considering the oxygen reduction potential of water in or exiting from a part of the reactor in determinig a nutrient addition amount or rate.
 7. The process of claim 3 having one or more inlets for diluting the liquid to the reactors, a system for adding lime or sulfides to the wastewater upstream of the reactors, or a precipitate remover.
 8. The process of claim 1 wherein lime, sulfides or combinations thereof are added in the pretreatment step.
 9. The process of claim 1 wherein the ammonia stripping takes place in a first aerobic zone. 